Phosphorus-based fertilizers are one of three critical nutrients for agriculture around the world. The others are nitrogen and potassium. All important phosphorus-based fertilizers comprise phosphate, e.g. phosphate ion (PO4−3), and occasionally some proportion of polyphosphates, i.e. P2O7−4, are included in the composition. Polyphosphates are ionic species formed by condensed phosphate ions (PO4−3) as illustrated in formula 1.

Polyphosphates are sometimes referred to as pyrophosphates. Additional phosphate ions may react further with the polyphosphate, P2O7−4, to form longer polyphosphates and, in general, there is a mixture of varying polymer chain lengths in any given sample. The presence of some proportion of polyphosphates in a fertilizer is useful for purposes of sequestration of impurities, as suspensions aids, and for making phosphorus more available to plants.
U.S. Pat. No. 3,917,475 describes a process for the preparation of a fertilizer containing a significant amount of polyphosphate. Formation of polyphosphate is favored by high concentrations of reactants and by high reaction temperatures, followed by rapid cooling. However, at high temperatures, the chemical bonds in polyphosphate can hydrolyze to yield the starting phosphate ion, (PO4−3) and acid, as shown in formula 2 (taken from Western Fertilizer Handbook, Interstate Publishers, Inc., Danville, Ill., Eighth ed., p148, 1985).

On the other hand, the lower valent phosphite, (PO3−3), has never played an important role in the commercial fertilizer industry.
A few academic research papers have been published describing alkali metal and organic esters of polyphosphites. For instance, Payne and Skledar of the University of Glaskow in J. Inorg. Nucl. Chem., 1964, Vol. 26, pp 2103-2111 describe the preparation of “pyrophosphates” by thermal decomposition of alkali metal phosphites. An academic paper from Russia in 1970 (CA 76:71456) describes a method of preparing ammonium polyphosphite by starting with phosphorous acid that has already been polymerized and reacting it with ammonia under pressure and at high temperature. Using polyphosphorous acid as a reactant to produce polyphosphites, however, is not economically feasible for commercial production.
The analysis of polyphosphite content in a composition is difficult because all common wet chemical methods for determination of phosphite depend upon reagents that first convert phosphite to phosphate. These reagents will break up any polyphosphite molecules present in the composition into individual phosphite ions. Polyphosphite, therefore, cannot be detected or quantified by the routine wet chemical methods. For instance, iodine solutions are used to oxidize inorganic phosphites for subsequent analysis as phosphate. Iodine will breakup any phosphite polymer present and the polyphosphite will not be detected. Similarly, commercial labs which analyze fertilizers do not report phosphite levels but rather report them as phosphate. Also, during analytical procedures requiring heat, phosphites would typically be slowly converted to phosphate unless precautions are taken to prevent oxidation by excluding air. Furthermore, at elevated temperatures polyphosphites can be expected to hydrolyze to ordinary phosphite ion, analogously to the hydrolysis of polyphosphates under similar conditions. Accordingly, physical methods such as nuclear magnetic resonance (NMR), high pressure liquid chromatography (HPLC), liquid chromatography, mass spectrometry (MS), and other physical molecular weight determining methods are useful methods for characterizing polyphosphites.
NMR provides a unique method of detecting phosphite because in most cases, and particularly when in solution, it exists with a hydrogen attached to the phosphorus atom (HPO3−2). Sophisticated NMR instruments, such as the Varian VXR-3005 spectrometer, can not only detect and measure P31 but can also simultaneously perform measurements on atoms such as hydrogen attached to phosphorus or carbon by transfer polarization. Such an instrument can, therefore, detect and measure phosphite in the presence of other phosphorus species without ambiguity.
Inorganic phosphite compositions such as potassium phosphite are known to be useful as fungicides, as described in U.S. Pat. Nos. 5,736,164, 5,800,837, and U.S. 2003/0029211A1, for instance. As is common with all commercial chemicals, however, and particularly so with environmentally sensitive chemicals such as fungicides, less is often better. Therefore, there is always a need for enhanced performance at an equivalent dose.
In this context, potassium phosphite would be particularly useful because it would provide the second important nutrient of the three critical plant nutrients, potassium. Moreover, a polyphosphite can be expected to provide the sequestration and slow release advantages known with polyphosphate, although phosphites are more active fungicides.
Currently available commercial methods for preparation of fertilizer grade potassium phosphite, KH2PO3 and/or K2HPO3, are carried out by charging an aqueous potassium hydroxide solution to a mixing tank equipped with an agitator and with cooling means (commonly called a batch reactor). Alternatively, potassium carbonate could be used as a reactant instead of potassium hydroxide. Phosphorous acid is added to the potassium hydroxide, slowly at first, then more rapidly toward the end of the reaction. This process is subject to a number of problems.
The reaction can be violent and on a large scale, even with good agitation and cooling, the reaction can run away explosively. In fact, at least two fatalities and numerous injuries have resulted recently from such run away reactions. During the early addition of phosphorous acid, even if the reaction does not run away, localized excessive heat release occurs, even when the over all temperature is at or below 200° F. Furthermore, it is known in the art that hazardous toxic phosphine gas, which has a characteristic garlic-like odor, may be emitted during the reaction when the temperature reaches 150° F., which creates a hazard unless properly absorbed. In addition, a batch reactor is difficult to seal and prevent oxygen in the air from entering, which readily oxidizes the phosphorous acid to phosphoric acid, preventing formation of phosphites.
The necessary slow addition of the acid results in the hydroxide always being in excess until close to the end of the reaction, thus hindering formation of the desired polyphosphite. As a consequence, and also due to low temperatures, previous processes can be expected to provide little or no formation of polyphosphites.
Yet another potential problem which occurs in batch processes is poor control of the addition rate of and total quantity of reactants present in the mixture. Extra care must be taken in measuring ingredients and in the rate of addition which is time consuming and labor intensive. Lack of attention by the technician can lead to an explosive run away reaction.
U.S. Pat. No. 3,585,020 by Legal, Jr., et al. describes a process for forming a free-flowing, granular, non-burning and non-crumbling 7-40-6 fertilizer composition. Reference is made at column 3 to the use of an inline mixer. However, the use of spargers in the process suggests that it is specific to batch processing. In any case, the reference by Legal, Jr., et al. is specific to forming granular materials quite different from the liquid solutions prepared in the present invention.
In U.S. Pat. No. 3,957,947 Yamada et al. describe a process for the continuous production of aqueous basic aluminum salt solutions. The products of Yamada appear to be deodorants, and while a short tubular reactor is involved, it is necessary to provide heat on an indirect basis and the overall reaction scheme is quite different from that of the present invention.
The Environmental Protection Agency, classifies potassium phosphite compositions as “biopesticides” under their regulatory classification, for reduced registration requirements. As such, the active ingredient, mono- and di-potassium salts of phosphorous acid are synthesized active ingredients involving a mixed mode of action by direct toxicity to plant pathogens, and by activating the plants natural defense mechanisms, in disease suppression or elimination.
Potassium phosphites are systemically absorbed by the plant and are mobile within the plant, translocating to the new growth via both the phloem and the xylem. They are rapidly absorbed by the leaf tissue and roots for maximum and efficient plant use by moving systemically upward and downward in the plants vascular system, including the root system. The mode of action is thought to be two-fold, first acting within the fungus by “walling off” the pathogen, killing off surrounding cells when attacked by disease or insects, and inhibiting further fungus growth. This is observed as yellowing around a diseased area. Secondly, the plant then responds further by activating the plant's own immune self-defense system, through rapid cytological action, and triggering other cellular phytoalexin accumulations and metabolic changes and other resistance inducers. Various chemical compounds are released that alert the rest of the plant to begin producing other compounds that increase plant resistance to infection or attack at other sites on the plant. These two types of responses are known as systemic acquired resistance (SAR) and induced resistance (IR).
As a result, phosphites are highly selective, non-toxic fungicides active against numerous fungal pathogens, and provide both protective and curative responses against such plant disease isolates of Phytophthora, Rhizoctonia, Pythium, and Fusarium, and other plant diseases—but typically not against bacterial diseases.
The extreme difficulty, or even the total lack of bacterial disease control, by induced systemic resistance compounds, including those based on the salts of phosphorous acid, and particularly the potassium salts, is well known. For example, the benchmark product, “Aliette”, a product comprising aluminum salts of phosphorous acid and EPA-registered pesticide, does not provide for any bacterial disease control.
Accordingly, the skilled will appreciate that a need exists for an economical and safer commercial process for the preparation of a potassium polyphosphite composition having enhanced effectiveness as an agricultural fungicide.